Although increased plasma levels of IgA1 and IgA1-containing immune complexes (IC) occur in several human diseases, their deposition into kidney mesangium is characteristic of IgA nephropathy (IgAN). Recent studies indicate that IgA1 proteins from plasma of IgAN patients are characterized by structurally altered oligosaccharide side chains. Carbohydrate analyses and reactivities with lectins revealed that IgA from IgAN patients is deficiently galactosylated and that this abnormality is restricted to the hinge region of IgA1 which contains 5 O-linked oligosaccharide side chains. The hinge region is primarily responsible for interactions of IgA1 with the asialoglycoprotein receptor (ASGP-R) on hepatocytes. The loss of hinge region galactose (Gal) results in increased plasma levels of IgA1, decreased binding to ASGP-R, but increased binding to kidneys and mesangial cells. Experimental approaches proposed in this application are designed to test the following hypothesis: An aberrant glycosylation pattern in a fraction of IgA1 proteins in IgAN patients leads to their altered elimination from the circulation and deposition in kidney mesangia. The following specific aims are proposed to obtain further evidence for this hypothesis: l) Perform comparative studies of carbohydrate moieties of serum and secretory IgA1 from IgAN patients and controls with respect to their reactivities with lectins and monoclonal antibodies specific for sialylated and non-sialylated forms N-acetylgalactosamine (Ga1NAc) and beta1-3 linked Gal; binding to receptors expressed on human hepatocytes, mesangial and monocytic cells; and carbohydrate compositions determined by a highly sensitive gas-liquid chromatography method. 2) Characterize the receptor(s) expressed on human mesangial cells specific for IgA1 molecules in various molecular forms having different glycosylation patterns. 3) Characterize IgA1 deposits in kidney biopsies from IgAN patients with respect to their reactivities with carbohydrate- specific lectins and monoclonal antibodies. 4) Provide evidence at the cellular level, for the secretion of IgA molecules with altered carbohydrate moieties by mitogen-stimulated and EBV-transformed peripheral blood lymphocytes from IgAN patients and healthy controls. 5) Correlate laboratory findings concerning the properties of aberrantly glycosylated IgA1 with the clinical course of the disease. Results of these studies should provide information concerning the behavior of structurally altered IgA1 proteins and provide a rational explanation for their deposition in kidney mesangium of IgAN patients.